1. Field of the Invention
The present invention relates to an ultra-thin magnetic device used for an inductor, a choke coil, a transformer and the like in electric equipment, and to a method for manufacturing the magnetic device and a power supply module equipped with the magnetic device.
2. Related Background Art
In recent years, with the general trend toward smaller and thinner electric equipment, there is a strong demand for smaller and thinner components, devices, power supplies and the like used in such electric equipment. Particularly, in the field of mobile equipment, the demand for making them thinner becomes stronger than that for making them smaller. Meanwhile, LSIs such as a CPU are now increasing in speed and density, and in some cases a large current is fed to a power supply circuit provided for such LSIs. Thus, a magnetic device used as an inductor or the like in the power supply circuit for such LSIs is required for: being constituted with a coil made of a conductor having low resistance, which realizes a low heating value; and suppressing a decrease in the inductance value due to direct current (DC) superimposition (i.e., having a favorable DC superimposition property). In addition, since operation frequencies tend to be higher, a small loss at high frequencies also is required. Furthermore, since it is required strongly to reduce the cost of components, elements constituting the components in a simple shape have to be assembled in a simple process. To sum up, it is required to supply an inexpensive magnetic device that is as small and thin as possible, which is operable with a large current and at high frequencies. Among components used in the power supply circuit, a magnetic device used as an inductor or the like is the thickest. Therefore, also in order to make the power supply itself thinner, the magnetic device is demanded strongly to be made thinner.
Generally, the miniaturization of magnetic devices decreases a cross-sectional area of the magnetic path, thus decreasing an inductance value of the device. As means for improving the property of such a miniaturized magnetic device (i.e., for increasing an inductance value), JP 53(1978)-136538 U and JP 61(1986)-136213 A, for example, suggest a magnetic device having a closed magnetic path structure formed by winding coils around a drum-shaped core with flanges made of ferrite or the like and by filling inside of the flanges with a mixture of a magnetic powder and a resin. This configuration can eliminate a bobbin, which is used with coils usually, and therefore a cross-sectional area of the magnetic path can be increased. In addition, by virtue of the closed magnetic path structure, the inductance value can be increased. In this way, properties of the magnetic device can be improved. However, the magnetic device with such a configuration has the following problems: that is, since this configuration aims to miniaturize the magnetic device, a device with a sufficient small thickness cannot be realized. In addition, low-permeability resin layers adhered to the outer surface of the magnetic device increase a leakage flux, resulting in insufficient properties. Furthermore, a special technology is necessary for shaping the resin layers adhered to the outer surface of the magnetic device. Although an inductor manufactured with such a technology and having a size of, for example, about 2xc3x971xc3x971 mm is now on the market, the coil constituting this inductor has large DC resistance.
In order to achieve a coil of low DC resistance and a large inductance value, the coil has to be manufactured with a thick wire and the number of turns also has to be increased. At the same time, in order to make a device thin, the thickness of the coil has to be made approximately 1 mm or less, but a cross-sectional area of the magnetic path has to be increased to some extent. To this end, it is preferable that the coil is wound not in solenoid form but in planar spiral form. In order to secure the space for accommodating the coil satisfying these conditions, the size of the device has to be increased to 2 to 10 mm square. However, such a thin configuration having a large area/thickness ratio increases a leakage flux, which makes the realization of a large inductance value difficult.
To cope with this problem, JP 58(1983)-133906 U, JP 59(1984)-67909 U, JP 1(1989)-157508 A, JP 1(1989)-310518 A and JP 3(1991)-284808 A, for example, suggest a configuration where a conductive coil wound in planar spiral form are sandwiched between ferromagnetic layers arranged on the upper and lower surfaces of the conductive coil with an insulating layer intervening therebetween. With this configuration, since high-permeability magnetic elements are disposed on the upper and lower surfaces of the conductive coil, a leakage flux therefrom can be made relatively small in the even thin configuration, which can realize a large inductance value. However, in the case of this configuration, the conductive coil is exposed at the side of the magnetic device, and therefore the device has a problem concerning the reliability. In addition, this configuration is uncertain as to a method for providing the adhesiveness between the respective parts.
As a magnetic device to cope with this problem, JP 59(1984)-23708 U and JP 6(1994)-342725 A suggest a configuration where a conductive coil wound in planar spiral form is embedded in a paste containing a mixture of a ferrite powder and a resin and ferrite boards are attached to the upper and lower surfaces of the paste. Also, JP 9(1997)-270334 A suggests a configuration where a conductive coil wound in planar spiral form is embedded in a resin containing a magnetic powder (hereinafter referred to as xe2x80x9cmagnetics containing resinxe2x80x9d) and thin metallic magnetic elements are attached to the upper and lower surface of the resin. With these configurations, since the conductive coils are embedded in the resin, the problem of the conductive coil being exposed at the side of the device does not occur. In addition, the ferrite boards and the thin metallic magnetic elements, which are disposed above and below the coil, can be bonded to the conductive coil embedded in a resin by curing the resin.
However, the magnetic device disclosed in JP 6(1994)-342725 A has a configuration where the conductive coil itself is embedded completely in the magnetics containing resin, which means that the magnetics containing resin is present between adjacent turns of the conductive coil and around the conductive coil. Therefore, magnetic paths functioning as a short path, which traverse within the conductor constituting the conductive coil or traverse across adjacent turns, are likely to occur, compared with the magnetic paths as what should be, which extend along the outer region of the conductive coil. Such an increase in the magnetic flux traversing in the conductor constituting the conductive coil and traversing across the conductors causes problems in that a magnetic loss is increased at high frequencies and at the same time the inductance value is decreased.
Furthermore, the magnetic devices disclosed in the above-mentioned publications have to be manufactured on a one-by-one basis, or with a vacuum process such as vacuum evaporation and sputtering, and therefore have problems of poor mass productivity and a high manufacturing cost.
A magnetic device according to the present invention includes a sheet-type coil including a planar conductive coil and an insulating substance; and a first magnetic member in sheet form disposed on at least one of upper and lower surfaces of the sheet-type coil. In this device, a magnetic permeability of the insulating substance is smaller than a magnetic permeability of the first magnetic member.
A method for manufacturing a magnetic device according to the present invention includes the steps of preparing a sheet-type coil including a planar conductive coil and an insulating substance; and then disposing a first magnetic member in sheet-form having a magnetic permeability larger than that of the insulating substance on at least one of upper and lower surfaces of the sheet-type coil.
A power supply module according to the present invention includes a wiring board and the magnetic device according to the present invention, which are connected electrically with each other.